Interesting.
I happen to have used the term 'sandbox' recently in a post here on the forums referring to my campaign, and I definately meant something else than mentioned here by the majority of the posters.
in my opinion, a 'sandbox' is where you try things out. There is no definiative statement as to WHAT you are trying out, however.
For some, it may be trying out different adventures without a bigger storyline behind them. For others, it might be even a campaign where the characters change between adventures, allowing the players to try out different characters to experience the entire spectrum available to them without being fixed to a character's lifespan.
For me, it's trying out houserules, changing the basic assumptions of the D&D universe, etc. I introduce new books and rules, and sometimes change them or retract them a few sessions later.
Normally, i would be hesitant to do this, since the rules are the very foundation on which we play. This particular campaign, however, seems to thrive on it. it's my 'sandbox', and as long as the sandcastles hold, I'll keep playing....
